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Inrush current protection for power supplies
Inrush current protection for power supplies
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Old 18th May 2018, 08:25 AM   #11
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by thoglette View Post
At the extremes, some of these required soft-start circuits but mostly it's not an issue for normal wiring and from-the-grid power
It's amazing how far beyond absurdity audiophile products tend to go.

From a little bit of reading I did on the subject of inrush currents, apparently a lot of today's switching power supplies use inrush current protection, because they contain large-value filter caps that are charged up rapidly by rectifying the incoming AC line voltage. There is no transformer in between, so there is negligible series resistance to limit inrush current.

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Old 18th May 2018, 08:46 AM   #12
JMFahey is offline JMFahey  Argentina
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Yes, offline switchers are a different beast.

Math is the same (of course) but diodes straight charge big filter caps, even before the transformer enters the equation, so *there* some kind of soft start is needed.
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 18th May 2018, 10:52 AM   #13
trobbins is offline trobbins  Australia
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Originally Posted by Gnobuddy View Post
- it was more a case of wondering what sort of long-term damage large inrush transient currents might inflict on filter caps and rectifier diodes.
Diodes can be a concern. Many appreciate that valve diodes definitely require care in selection. And the UF4007 is not as well rated as the 1N4007, although a valve amp would need to be pretty potent to require more. ss amp peak currents are a little harder to define as circuit and part resistances are much lower.

Filter caps are typically only a concern from a ripple current perspective. i've not come across any degradation data relating inrush for common filter caps.
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Old 26th July 2018, 07:33 AM   #14
bondini is offline bondini  Australia
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Inrush current protection for power supplies
Originally Posted by trobbins View Post
Diodes can be a concern. Many appreciate that valve diodes definitely require care in selection.
Yep - depending on the design, a valve rectified power supply that delivers steady current well below the maximum specified in the data sheet can nonetheless draw peak plate current well above the specified maximum during the first half second or so after switch-on. A bit of modelling using PSUD2 can reveal this even though the power supply seems basically OK.

I have seen this with guitar amps - the new rectifier works well for a number of gigs and then "phut" it dies on start up. Guitarists take to carrying a spare. One possible solution is to include a suitable NTC resistor on the transformer primary to reduce the inrush current at switch-on. Now I can't say for certain that this works, as I've not checked to see if the rectifiers still go "phut". But, I've not heard back and assume that either 1) it worked and the guitarist has no need to talk to me again or 2) it didn't work the guitarist doesn't want to talk to me ever again.
In theory, there is no difference between theory and practice. But, in practice, there is.
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